Aptitude - Problems on Trains - Discussion

36 - 20 = 16,
54 * 5/18 = 15,
16 * 15 = 240.

Why we multiply 15*20?

@ALL.

Logically, the length of the train should be more or less equal to the length of the platform else it will be troublesome for the passengers to cover that distance. And the solution assumes that the man is standing at the beginning of the platform.

For finding length of the train we must consider a reference point (man).

But for finding the distance covered why do we need the length of the train.

Simply we can do DISTANCE COVERED = SPEED X TIME.

Can anyone explain it, whether this is correct method are wrong?

@Harish.

Why do we need the length of train?"
=> when the question says " A train passes a station platform in 36 seconds" it means the time between when an engine of the train has crossed the starting point of platform till trailer had passed the end point of the platform.

If you actually analyse this, we find that train had already passed platforms total length(x) when its engine crossed end point of platform and it is still going on till its trailer crosses end point of platform(i.e. train went 300m extra which is its own length)
Trains pass length = x+300;
Trains pass time= 36 sec.

Therefore we substitute (distance) = (speed) * (time).
(x+300)= (54 * 5/18) * 36.
=> x =240m.

Time ratio of train A & B is 27 : 17.
Hence speed ratio of A & B respectively is 17:27.
They meet after 23 sec ,hence 23-17 =6 & 27-23 =4.
Then ratio is 6 : 4 means 3 : 2.

If you can think then,

(36-20) * 54000/3600 = 240m.

I did it like this.

First, while train passing thru platform in 36 sec = 15 x 36 = 540,
Second while train passing thru man in 20sec = 15 x 20 = 300,
Now, the relative length 540 - 300 = 240.

Thanks @Satheesh.

When the train enters fully from where the platform begins and at the beginning point where the man is standing it takes 20 sec so,

=54*5/18*20= 300 and that is the length of the train.